Ethylene oxide is an important basic chemical and frequently prepared on an industrial scale by direct oxidation of ethylene with oxygen in the presence of silver-containing catalysts. These catalysts usually comprise metallic silver and further elements, which have been deposited on a support material by means of a suitable process. As supports, it is in principle possible to use various porous materials such as activated carbon, titanium dioxide, zirconium dioxide or silicon dioxide or ceramic compositions or mixtures of these materials. In general, alpha-aluminum oxide is used as support.
Apart from silver as active component, these catalysts often comprise promoters for improving the catalytic properties (WO 2007/122090, WO 2010/123856). Examples of promoters are alkali metal compounds and/or alkaline earth metal compounds. Some documents teach the use of transition metals such as cobalt (EP 0 480 538), tungsten or molybdenum. A particularly preferred promoter for influencing the activity and selectivity of catalysts is rhenium (EP 0 266 015). In industry, preference is given to using catalysts comprising rhenium and/or other transition metal promoters in combination with alkali metal compounds and/or alkaline earth metal compounds because of their high selectivity. Selectivity is, for example in the case of the oxidation of ethylene, the molar percentage of ethylene which reacts to form ethylene oxide. The activity of the catalyst is usually characterized by the ethylene oxide concentration at the reactor outlet under otherwise constant conditions, for example temperature, pressure, gas throughput, amount of catalyst, etc. The higher the ethylene oxide concentration in the reactor output stream, the higher the activity of the catalyst. The lower the temperature required for achieving a predetermined ethylene oxide concentration, the higher the activity.
The direct oxidation of ethylene to ethylene oxide using supported silver catalysts is described, for example, in DE-A-2300512, DE-A 2521906, EP-A-0014457, DE-A-2454972, EP-A-0172565, EP-A-0357293, EP-A-0266015, EP-A-0011356, EP-A-0085237, DE-A-2560684 and DE-A2753359.
EP 1 613 428 B1 describes the production of ethylene oxide from ethylene using a catalyst containing rhenium in an amount of at most 1.5 mmol/kg with respect to the total weight of the catalyst or 0.0015 mmol/m2 with respect to the BET surface area of the support. EP 2 152 411 A2 describes the use of promoters and co-promoters consisting of sulfur, phosphorus, boron, or mixtures thereof, tungsten, molybdenum, chromium, such that the quantity of the co-promoter deposited on the support is at most 3.8 mmol/kg relative to the weight of the support.